Photopolymerizable compositions generally comprise an addition polymerizable compound having at least two ethylenically unsaturated bonds in the molecule (hereinafter referred to as "polyfunctional monomer") and a photopolymerization initiator, and are insolubilized in solvents by curing upon irradiation with light. Taking advantage of this property, they have been widely used in the fields of photography, printing, metal surface finishing, inks, and the like. Descriptions thereof appear in J. Kosar, Light Sensitive Systems, pp. 158-193, J. Wiley & Sons (New York, 1965).
Intensive studies have hitherto been made in an attempt to increase light sensitivity of the photopolymerizable compositions. For example, various photopolymerization initiators have been proposed, such as benzoin ethers as described in U.S. Pat. No. 2,448,828, benzoins as described in U.S. Pat. No. 2,722,512, anthraquinones as described in U.S. Pat. No. 3,046,127, combinations of aminophenyl ketones and active methyl compounds or amino compounds as described in Japanese Patent Publication No. 11936/74, combinations of Michler's ketone and benzophenone as described in U.S. Pat. No. 3,682,641, etc. Although these conventional photopolymerization initiators realize improvement of light sensitivity, they are still unsatisfactory in practical use. In particular, when they are used in a light-sensitive layer of a light-sensitive element, such as a light-sensitive dry film resist for producing light-sensitive lithographic printing plates or printed circuit boards, a print-out function is imparted to a light-sensitive layer in order to clearly distinguish between exposed areas and unexposed areas immediately upon exposure to light. To this effect, a leucotriphenylmethane compound or a dye whose color disappears under acidic conditions is usually added to a light-sensitive composition. However, resist-forming sensitivity is frequently inhibited by such additives. Further, a heat polymerization inhibitor is added for the purpose of ensuring heat stability of the light-sensitive layer, but this also causes reduction in sensitivity for resist formation. Accordingly, it has been demanded to develop a photopolymerization initiator that provides a highly sensitive photopolymerizable composition.
It is taught in U.S. Pat. No. 2,773,822 that thiol compounds are effective as photopolymerization initiators. It is also known to use thiol compounds in combination with other photopolymerization initiators to enhance sensitivity. For example, U.S. Pat. No. 3,479,185 describes that sensitivity can be increased by using 4-acetamidothiophenol, 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, or 2-mercaptosuccinic acid in combination with a lophine dimer. Japanese Patent Application (OPI) No. 702/78 (the term "OPI" herein used means "published unexamined application", hereinafter the same) describes that mercaptoimidazoline, etc., is effective not only to accelerate adhesion of a resist to a copper base but also to increase sensitivity. Further, Japanese Patent Application (OPI) No. 56403/84 discloses a combination of a p-dialkylaminostilbene derivative, a lophine dimer and mercaptobenzoxazole. According to the above-cited patents and applications, the effect of these mercapto compounds to increase sensitivity is attributed to prevention of oxygen from inhibition during exposure and polymerization.
Addition of the above-described known compounds to photopolymerizable compositions surely brings about an increase in sensitivity, but such increased sensitivity is only about twice the sensitivity obtained in the absence of these compounds at the most and is still unsatisfactory in level.